1. Field of the Invention
The present invention relates to electrical safety products, more particularly, to methods, fabric structures, and assemblages for providing an arc flash barrier between workers and electrical equipment.
2. Description of the Related Art
Flame resistant (FR) clothing and blankets are used by electrical workers and electricians to protect themselves from exposure to the thermal effects of an electric arc flash. The heat from an electric arc flash can be extremely intense and is accompanied by a shock wave due to the rapid heating of the air and gases in the vicinity of the arc flash.
Protective clothing systems called arc flash suits have been developed to protect workers who may be exposed to an arc flash. Suits are designed to provide protection for various levels of exposure. The combination of radiant and convective heat and the shock wave can cause breakopen of the FR fabrics in an arc flash suit as well as heat transmission through the suit to the wearer. Breakopen is defined as the creation of holes, tears, or cracks in the exposed fabric such that incident energy is no longer effectively blocked by the fabric and passes to surfaces below the fabric. Breakopen in the ASTM F1959 test method is defined as a hole in the fabric specimen with an area greater than 0.5 square inch or a crack in any dimension greater than 1 inch in length. Arc flash suits are tested according to ASTM F1959-99 and assigned an arc rating based on the level of protection that the suit can provide. The arc rating is specified as heat energy per unit area using units of calories per square centimeter (cal/cm2) and is defined using one of two criteria, which ever occurs at the lower incident heat energy. The first criteria is the level of incident heat energy from an arc exposure that would cause a 50% probability of a second degree burn injury for the wearer. This is expressed as the arc thermal performance value (ATPV). The second criteria is the level of incident heat energy from an arc exposure at which breakopen of all the FR fabric layers in the system is just beginning to occur. This is expressed as the breakopen threshold energy (EBT).
Arc flash suits with moderate to high arc ratings of protection from 25 cal/cm2 to over 100 cal/cm2 are comprised of multiple layers of FR fabrics. FR fabrics include flame retardant treated cotton fabrics and cloth woven from yarns containing predominantly a para-aramid fiber, such as Dupont's KEVLAR®, and/or a meta-aramid fiber, such as Dupont's NOMEX®. Greater protection requires more fabric layers, with a proportionate increase in garment weight, discomfort, necessary storage space, and cost.
Arc protective blankets and curtains provide secondary protection for workers and equipment. The blanket or curtain is hung between the equipment that is recognized to have significant arc hazards or equipment that is being worked on and the individual and/or other areas to be protected. These blankets are composed of one or more layers of FR fabrics, the number of layers and type of fabric determining the amount of protection provided. As with the arc flash suits, the greater the protection provided, the greater the blanket weight, necessary space, and cost.